fix(gemini): relax validation when adding providers (#210)

Allow users to create Gemini provider configurations without API key
and fill it in later. Split validation into two modes:

- validate_gemini_settings: Basic structure check (used when adding)
- validate_gemini_settings_strict: Full validation (used when switching)

This fixes the error 'Gemini config missing required field: GEMINI_API_KEY'
when trying to add Gemini providers from presets like PackyCode or Google.

Changes:
- Add validate_gemini_settings_strict for switching validation
- Update write_gemini_live to use strict validation
- Add comprehensive tests for both validation modes

src-tauri/src/services/provider.rs

@@ -1465,7 +1465,7 @@ impl ProviderService {
 
     fn write_gemini_live(provider: &Provider) -> Result<(), AppError> {
         use crate::gemini_config::{
-            json_to_env, validate_gemini_settings, write_gemini_env_atomic,
+            json_to_env, validate_gemini_settings_strict, write_gemini_env_atomic,
         };
 
         // 一次性检测认证类型，避免重复检测
@@ -1479,15 +1479,15 @@ impl ProviderService {
                 Self::ensure_google_oauth_security_flag(provider)?;
             }
             GeminiAuthType::Packycode => {
-                // PackyCode 供应商，使用 API Key
-                validate_gemini_settings(&provider.settings_config)?;
+                // PackyCode 供应商，使用 API Key（切换时严格验证）
+                validate_gemini_settings_strict(&provider.settings_config)?;
                 let env_map = json_to_env(&provider.settings_config)?;
                 write_gemini_env_atomic(&env_map)?;
                 Self::ensure_packycode_security_flag(provider)?;
             }
             GeminiAuthType::Generic => {
-                // 通用供应商，使用 API Key
-                validate_gemini_settings(&provider.settings_config)?;
+                // 通用供应商，使用 API Key（切换时严格验证）
+                validate_gemini_settings_strict(&provider.settings_config)?;
                 let env_map = json_to_env(&provider.settings_config)?;
                 write_gemini_env_atomic(&env_map)?;
             }